Apparatus and method for obtaining high temperature low pressure vapor from a high temperature high pressure vapor source



March 29, 1966 E. L. KOCHEY, JR

APPARATUS AND METHOD FOR OBTAINING HIGH TEMPERATURE LOW PRESSURE VAPOR FROM A HIGH TEMPERATURE HIGH PRESSURE VAPOR SOURCE Filed Sept. 20, 1963 2 Sheets-Sheet 1 March 29, 1966 Y, JR 3,242,678

APPARATUS AND METHOD FOR OBTAINING HIGH TEMPERATURE LOW PRESSURE VAPOR FROM A HIGH TEMPERATURE HIGH PRESSURE VAPOR SOURCE Filed Sept. 20, 1963 2 Sheets-Sheet 2 FIG. 2

United States Patent APPARATUS AND METHOD FOR OBTAINING HIGH TEMPERATURE LOW PRESSURE VAPOR FROM A HIGH TEMPERATURE HIGH PRES- SURE VAPOR SOURCE Edward L. Kochey, Jr., Colebrook, Conn., assignor to Combustion Engineering, Inc., Windsor, C0nn., a corporation of Delaware Filed Sept. 20, 1963, Ser. No. 310,312 Claims. (Cl. 60104) This invention relates to vapor power plants and in particular to a method and apparatus for obtaining steam at high temperature and low pressure at the turbine during startup.

During operation of a steam turbine the internal structure assumes the temperature of the steam at that location, such as 1000 F. at the inlet of turbines in modern power plants. When a turbine is removed from operation, it takes a matter of days for the metal temperature inside the turbine to approach room temperature. It is often desired that the turbine be restarted before this cooldown can be effected.

If the temperature of the steam introduced through and against the hot internal surfaces is considerably below the temperature of the structure at any time, large thermal stresses are set up due to differential expansion resulting from temperature difference through the structure. The stresses actually come about due to temperature differences within the mass of metal so that portions of the mass tending to expand or contract are constrained by the remainder of the mass. The differential temperature that exists at this time is that between the bulk of the material and the inner surface which is approaching the steam temperature. The magnitude of these stresses is a function of the thickness of the parts, of the configuration of the parts, and the temperature difference between the steam and the metal as well as a time effect dependent on the resistance of the internal film to heat transfer and the conductivity of the material.

Modern turbines for high pressures are designed with walls of such thickness that this problem has become of critical importance. Although these large stresses could be taken occasionally, repeated cycling induces fatigue cracking.

Furthermore, the load requirements on power plant systems are increasingly calling for units which can be shut down either over the weekend, or in some cases simply overnight. The frequency with which these units must be restarted aggravates the cracking problem.

It can readily be seen that there is considerable advantage to supplying high temperature steam to the internal parts of the turbine when the turbine is hot. Where high temperature steam is supplied at high pressure ahead of the turbine throttling valve, the throttling temperature drop results in relatively low temperatures inside the turbine. For instance if steam is supplied at 3500 p.s.i. and 900 F'., throttling to 200 p.s.i., as would occur in passing through a turbine throttle valve during startup, would result in a steam temperature of only 650 F. Steam supplied at 2500 p.s.i. and 900 F. would drop to about 700 F. when throttled to 200 p.s.i.

In the case of a recirculating type boiler, the boiler will remain at a high pressure for a matter of hours after tripout. Unless maintenance work must be done on the boiler proper, it would not normally be cooled down. The heat removed in cooling a boiler down must necessarily be replaced during startup with consequent increased fuel consumption. Rapid depressurizing and, therefore, cooling of the boiler imposes stress problems in its own structure which are similar to those of the turbine and is, therefore, not desirable.

ice

Once-through boilers of either the subcritical or supercritical type have minimum acceptable operating pressures due to the characteristics of steam as they relate to boiler design. Therefore with either type boiler it is normal to have a high pressure existing in the boiler during startup.

During the startup of a steam generator the condition exists where small amounts of fuel are burned in a furnace structure which is designed for a high heat release. It is difiicult to obtain high gas temperatures at this time and, therefore, difficult to obtain high steam temperatures. During some phases of the startup, it may be possible to operate the furnace in such a manner as to obtain high gas temperatures but limitations on boiler tubing metal temperatures, particularly in the case of reheater tubing with no flow through it, put definite limits on the allowable gas temperature.

It can thus be seen that during startup it is difiicult enough to obtain high steam temperatures at the boiler outlet and that furthermore this steam is at high pressure. In throttling this steam to the low pressure existing inside the turbine at this time, the temperature drops considerably imposing high thermal stresses when the turbine surfaces are hot.

Briefly, myinvention comprises an apparatus and method for supplying relatively high temperature steam to the turbine. This is accomplished by passing the high pressure steam through one side of a heat exchanger and then throttling the steam to a desired lower pressure with a consequent reduction in temperature. A portion of this steam is then passed through the secondary side of the same heat exchanger, thereby raising the temperature of this portion of the steam to approach that of the initial high pressure steam. By this means steam is available to the turbine at higher temperature and the difiiculties encountered in supplying low temperature steam to the hot turbine surfaces are relieved.

It is an object of this invention to provide an improved turbine startup method and apparatus.

It is a further object to achieve this by supplying low pressure steam at high temperature to the turbine during startup.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawings wherein:

FIGURE 1 is a schematic of a power plan system embodying the present invention and wherein a heat exchanger organization is disposed in a turbine bypass line; and

FIGURE 2 shows a schematic in the nature of that of FIGURE 1 but depicting a modification where the heat exchanger organization is located in the main steam line.

Referring now to the drawings, FIGURE 1 shows a power plant system where condensate is pumped from condenser 2 by condensate pump 4 through low pressure feedwater heaters 6 to the de-aerator 8, where oxygen is removedby direct contact with steam supplied from a turbine extraction point (not shown) through pipe 30. Feedwater pump 10 takes it suction from the de-aerator, pumping the water through high pressure feedwater heaters 12 and feedwater valve 14 into steam generator 16. The steam generated therein passes through steam line 18 and turbine throttle valve 20 supplying the high pressure turbine 22.. The steam then passes through reheater 24, intermediate pressure turbine 26 and low pressure turbine 28 being condensed in con-denser 2.

This cycle, so described, is the normal operating cycle of the power plant, as it operates free of the startup system. Turbines 22, 26 and 28 form a unit driving an electric generator (not shown).

During startup, feedwater follows the same flow path from the condenser 2 to steam generator 16. The boiler eflluent is then passed through steam line 18 and heat exchanger 34 entering flash tank 38 through throttling valve 36. Depending on the enthalpy of the boiler effluent and the pressure in the flash tank, varying proportions of water and steam will be formed. It the boiler is cold at this time, no steam will be formed in the flashtank. Water is removed from the flash tank by valve 40, which operates to hold a water level in the tank, with the removed water passing through pipe 42 and returning to condenser 2.

When the temperature of the boiler efiiuent increases, steam becomes available in the flash tank and may be used to supply to de-aerator 8 through pipe 44. Excess steam formed in the flash tank will be returned to the condenser via pipe 44 and valve 46.

In order to roll the turbine, comprising 22, 26 and 28, steamis admitted through valve 48 passing through the low pressure side of the heat exchanger 34 and thence through pipe 49 into the turbine chest. The temperature of the steam thus entering the turbine will thereby be increased from the throttled temperature, approaching the temperature of the boiler effluent.

By firing the steam generator the eflluent temperature may be increased and controlled in an orderly manner during the startup. This temperature is controlled in such a manner that in conjunction with the heat exchanger, steam of the desired temperature is admitted to the turbine. As the temperature of this boiler eflluent is increased, the flash tank will normally go dry; that is, no water will be flashed and only steam will be produced in the flash tank. As previously described, any steam beyond that used will be dumped through valve 46.

It should be noted that in a case where the steam passing through the high pressure side of the heat exchanger 34 is all passed through valve 48 and through the low pressure side of the heat exchanger there will be no gain in temperature over that which would occur with ordinary throttling through valve 20. However, where water is removed from the flash tank or where steam is passed through pipe 44 the steam flow through the low pressure side of the heat exchanger is less than the flow through the high pressure side, and therefore there will be a net gain in steam temperature. The steam temperature supplied to the turbine from the heat exchanger will be a function of the temperature of the boiler effluent, the pressure held in the flash tank, particularly when saturated conditions exist, and the ratio of high pressure to low pressure flow through the heat exchanger. Manipulation of these variables enables control of the temperature of steam supplied to the turbine. Within the ability of the steam generator 16 the temperature of the effluent may be controlled with the normal boiler controls. The

flow relationship within the heat exchanger may be controlled by varying the steam supply from the boiler and controlling valve 46 to spill excess steam. Except in cases where steam demand for the de-aerator exceeds the available supply, the pressure in the flash tank may be controlled by operation of valve 46.

While operating the turbine in this manner, it is brought up to speed and the generator is synchronized. At this .point we have a nominal load on the unit. Load may be increased through this system until the capacity of the heat exchanger system is approached, and at this point .steps must be taken to open main control valve 20.

Valve 20 is simply opened and as the pressure in the turbine chest builds up, the flow through the heat exchanger will be inherently cut back. To be sure. the

chest valve system.

4 steam temperature in the turbine chest will drop at this time; however, since the. boileris operating at a higher load than during initial startup, it is able to produce relatively high steam temperature. Also, since the generator is already synchronized, load may be rapidly increased at this time, and the time period of low temperature operation will be relatively short. Passing quickly through this operation the material inside the turbine will not have time to change temperature extensively and, therefore, will not develop the high thermal stresses. I FIGURE 2 shows an application where 18 is again the steam line which, in this embodiment, supplies steam through heat exchanger 50 and main control valve 20 to turbine 22 during normal operation. During startup the boiler effluent is extracted through pipe 52 and throttling valve 36 passing into flash tank 38. Water is again removed through valve 40 and steam passes in a manner similar to the first embodiment through pipe 44. In order to roll the turbine steam is admitted through stopcheck valve 48 passing through the low pressure side of 'heat exchanger 50 and thereby supplying steam to turbine 22 through 49. Operation of this system is identical to the system shown in FIGURE 1.

The system of FIGURE 2 has some advantage over that of FIGURE 1 when valve 20 is first opened. The fact that the steam now passing through valve 20 also passes through heat exchanger 50 the proportion of high pressure flow to low pressure flow in the heat exchanger is maintained at a high level thereby increasing the temperature of any low pressure steam which is still leaving the heat exchanger. It, however, has the disadvantage that the heat exchanger being in the main flow path imposes a pressure drop on the plant system during normal full load operation with a consequent degradation of heat rate. A

While I have illustrated and described preferred embodiments of my invention it is to be understood that as to vapor, and it is intended that the term gas used in the claims include vapor; also valve 48 could equally well be inserted after the heat exchanger in such a manner that it may be an integral part of the turbine steam I therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes as fall within the purview of my invention.

What I claim is: p 1. An apparatus for supplying high temperature, low pressure gas to a turbine from a high temperature, high pressure gas source comprising: v v

a heat'exchanger connected to receive as its, heating medium, gas from the-gas source,

variable throttling means downstream of the heating side of the heat exchanger to reduce the pressure of at least some of the gaspassing therethrough; v H

means for conveying a portion of the gas through th e heat exchanger as the heated medium so as m ncrease the temperature of this portion of gas, and

means for conveying the portion of throttled gas from the heat exchanger to the turbine.

2. A power plant system having in combination:

a high pressure gas source,

a turbine connected to receive gas from the gas source,

a heat exchanger having a high pressure and a low pressure side, I

means for conveying the gas from the gas source through the high pressure side of the heat exchanger,

a throttling means,

means for conveying the gasfrom the high pressure side of the heat exchanger to the throttling means, means connected downstream of the throttling means for conveying a portion of the gas passing through the high pressure side of the heat exchanger, through the low pressure side of the heat exchanger, so as to increase the temperature of the portion of the throttled gas,

means for conveying this portion of gas to the turbine after traversal of the low pressure side of the heat exchanger,

a main control valve, and

means including the main control valve for conveying unthrottled gas from the gas source to the turbine.

3. An apparatus for supplying high temperature, low pressure vapor to a vapor turbine from a high temperature, high pressure vapor generator comprising:

a heat exchanger connected to receive as its heating medium vapor from the vapor generator,

means operative to throttle this vapor after traversal of the heat exchanger,

a flash tank receiving the throttled vapor,

means for conveying at least a portion of the throttled vapor through the heat exchanger as the heated medium, and

means for conveying said portion of the vapor from the heat exchanger to the turbine.

4. A system as in claim 3 including a control valve located between the flash tank and the heat receiving side of the heat exchanger.

5. An apparatus for supplying high temperature, low pressure gas to a turbine from a high temperature, high pressure gas source comprising:

a main control valve,

means for conveying gas from the gas source to the turbine including the main control valve,

a startup system in parallel relation with the control valve including:

a heat exchanger having a high pressure and a low pressure side,

a variable throttling means,

means for conveying gas from the first mentioned means at a location upsteram of the main control valve through the high pressure side of the heat exchanger and thence through the throttling means,

means for conveying the gas from the throttling means through the low pressure side of the heat exchanger connecting to the first mentioned means at a location downstream of the main control valve,

means for removing gas from the startup system at a location between the throttling means and the low pressure side of the heat exchanger.

6. An apparatus as in claim 5 including a startup control valve located in the means for conveying the gas from the throttling means to the first mentioned means at a location downstream of the main control valve.

7. A system for supplying gas to a turbine during startup comprising:

a high pressure gas generator,

a heat exchanger having a high pressure and a low pressure side,

means for conveying gas from the gas source to the turbine including the high pressure side of the heat exchanger,

a main control valve located downstream of the high pressure side of the heat exchanger,

a startup system in parallel relationship with the main control valve including:

a throttling means,

means for conveying gas from a location in the first mentioned means between the high pressure side of the heat exchanger and the control valve, through the throttling means,

thence through the low pressure side of the heat exchanger, and thence joining the first mentioned means at a location between the control valve and the turbine,

means for removing gas from the startup system, connected to the startup system at a location between the high pressure side of the heat exchanger and the low pressure side of the heat exchanger.

'8. A method for supplying high temperature, low pressure gas to a turbine from a high temperature, high pressure gas source comprising:

establishing a flow of the high pressure gas at an elevated temperature,

throttling this gas to a desired lower pressure, conveying a portion of this throttled gas in heat exchange relationship with the high pressure gas at elevated temperature, to raise the temperature of this throttled gas, and

thereafter conveying this portion of throttled gas to the turbine.

9. A method for supplying high temperature, low pressure gas to a turbine from a high temperature, high pressure gas source comprising:

conveying gas from the gas source through the high pressure side of a heat exchanger,

reducing the pressure of the gas,

conveying a portion of the throttled gas through the low pressure side of the heat exchanger, and delivering this portion of the gas to the turbine. 10. A method for starting up a steam turbine comprising:

conveying efiluent from a steam generator through a first side of a heat exchanger, thence through a throttling means into a flash tank, removing water from the flash tank to a condenser,

simultaneously firing the steam generator to raise the temperature of the efiluent, removing steam from the flash tank as the temperature of the eflluent increase, and passing at least a portion of the steam through a second side of the heat exchanger and thence to the turbine, and

subsequently supplying steam from the steam generator to the turbine through a direct means excluding the flash tank circuit.

N 0 references cited.

SAMUEL LEVINE, Primary Examiner.

R. R. BUNEVICH, Assistant Examiner. 

10. A METHOD FOR STARTING UP A STEAM TURBINE COMPRISING: CONVEYING EFFLUENT FROM A STEAM GENERATOR THROUGH A FIRST SIDE OF A HEAT EXCHANGER, THENCE THROUGH A THROTTLING MEANS INTO A FLASH, REMOVING WATER FROM THE FLASH TANK TO A CONDENSER, SIMULTANEOUSLY FIRING THE STEAM GENERATOR TO RAISE THE TEMPERATURE OF THE EFFLUENT, 